The proxy device documentation (doc/reference/devices_proxy.md) currently uses a static table of valid options. This should be moved to an auto-generated table using gendoc.

This is typically done in two steps:

1. Add the required gendoc comments where the configuration options are defined (usually alongside their validator)
2. Generate the new config metadata (make update-metadata)
3. Update the documentation to use the generated metadata

See #847 for an example.